tools: fix the wrong check when use virsh setvcpus --maximum

tools: fix the wrong check when use virsh setvcpus --maximum

The --maximum option wasn't properly parsed and the equivalent flag
wasn't set.  Fix this bug and also rewrite the way we check this option
by using new macro.  The new approach is that --maximum requires
--config, no other combination is allowed, because they don't make sense.

The new error will be:

 # virsh setvcpus test --maximum 10
 error: Option --config is required by option --maximum

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1204033

Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>